UREDINALES 563 



plant usually far removed systematically from the host with the aecial 

 uninucleate mycelium. Uredinales of this type, which for the completion 

 of their life cycle must successively infect two different host species, are 

 called heteroecious. 



In some genera the method of germination of aeciospores is vari- 

 able. In Endophyllum Sempervivi, they form germ tubes as usual 

 when covered with water; in damp air, they germinate with another 

 form of fructification, as basidia and basidiospores. The basidiospores 

 are again able to infect individuals of Sempervivum, while infection by the 

 usual germ tubes has not succeeded. Similarly, the aeciospores of Gymno- 

 conia Peckiana (G. inter stitialis) on Rubus, especially late in the season, 

 germinate occasionally with basidia instead of with germ tubes (Kunkel, 

 1920). Also in Kunkelia nitens on Rubus, germination normally takes 

 place with basidia and basidiospores; in certain strains of Gymnoconia 

 Peckiana, a caeoma of the Gymnoconia type (tube germination) and one of 

 Kunkelia type (basidiospore germination) appear on the same mycelium ; 

 indeed, within the same caeoma both spore types may appear (B. O. 

 Dodge, 1923). 



On the binucleate mycelium which arises from the aeciospores 

 germinating with a germ tube, aecia may again arise in some forms, 

 e.g., in the above mentioned species: Puccinia Senecionis, Uromyces 

 Hedysari-obscuri and Phragmidium disciflorum. The cytological proc- 

 esses which take place in them have been discussed by Kursanov (1916) 

 in Uromyces Scrophulariae on Scrophularia nodosa and U. Behenis on 

 Silene Cucubalus (S. inflata). After infection by aeciospores had been 

 completed, both uni- and binucleate mycelium was observed in the host 

 plants; the binucleate mycelium has apparently resulted from the uninu- 

 cleate mycelium by a pseudogamous plasmogamy. Both types of mycelium 

 intermingle; nevertheless the pycnia are formed by uninucleate hyphae. 

 In the hyphal knots one also finds binucleate hyphae, but the basal 

 cells of the pycnospores are entirely uninucleate. The aecia are laid 

 down at the same time as the pycnia. In the primordial knots uninu- 

 cleate hyphae sometimes predominate, at other times binucleate. The 

 binucleate hyphae proceed apogamously to the direct formation of 

 aeciospore mother cells; between the uninucleate hyphae an isogamous 

 sexual act takes place, and aeciospore mother cells are formed in the 

 normal manner. Somewhat later, at the same point of infection which 

 has already borne pycnia and aecia, a telium is laid down exclusively 

 by binucleate hyphae whose end cells change in a normal manner to 

 the teliospore mother cells. 



If one again infects the host with aeciospores formed in the first 

 generation, one obtains a second generation of the fungus normally 

 binucleate but nevertheless forming aecia (and later telia, but never 

 pycnia) in a normal manner. The teliospores are identical with those 



